Welcome back. In this lesson we're going to talk about the classification of polymers. First of all, the easiest classification or one of the most important is the degree of polymerization. That tells us how long the polymer chain is. In addition to having the polymer chain itself described, we also talk about the types of bonding that we have between polymer chains. As a result, we can have two types. One of them involves only secondary bonds. Weak secondary bonds between polymer chains. The other is when we have a cross-link between polymer chains and they become connected to one other through this cross-linking. We can also talk about the molecular weight of a polymer and when we do that we have to talk about a couple of different parameters. One of them is the number average, molecular weight and the other one is the weight average molecular weight. And so what that's telling us is that the smaller polymers, in terms of the distribution, they're going to wind up having a larger effect on the number average. Where when we start looking at the higher molecular weight sections of polymer chains, we're going to see that's going to have an effect on the weight average molecular weight. If we take those two parameters, the weight average and the number average, and we take and divide the weight average by the number average, we get something we refer to as the polydispersity. That tells us a good deal about the overall distribution of polymers in the system. Another important characteristic is what we refer to as the chain architecture. When we talk about that we have two different categories that we need to describe. The first one is confirmation. And, what we mean by confirmation is the fact that the carbon carbon, bond angle is 109.5 degrees. However, that doesn't necessarily lead to a polymer chain that is In a straight line and on a single plane, so we're going to see that the confirmation allows for the polymer to coil up. The other possibility in terms of the architectures is something we referred to as the configuration. And when we talk about configuration we're talking about tacticity and we're talking about side branches. And we'll see these and examples of all of these classifications in the succeeding lessons that we'll be describing. Thank you.